1. Field of the Invention
The present invention relates to a spread spectrum communication system, and particularly relates to a spread spectrum communication system using an error correcting code.
2. Description of the Related Art
Since networking in offices, factories, etc. has rapidly become popular, not only the number of users to be connected to a network but also the amount of data to be transferred over the network has been increasing. Normally, an information transmission speed is improved to cope with the increase in the amount of data.
When information is transmitted, transmission data is normally modulated. That is, the data is modulated and transmitted on a transmitting side, and the modulated data is demodulated on a receiving side. As the modulation method, the PSK (Phase Shift Keying), the QAM (Quadrature Amplitude Modulation), etc. are known.
Provided hereinafter is the explanation about the technique for improving an information transmission speed by adopting a system using a spread spectrum communication system as a system for transmitting modulated data. As the spread spectrum communication, the DS (Direct Sequence) method, the FH (Frequency Hopping) method, etc. are known. Here, the DS method is to be described.
FIG. 1A is a schematic diagram showing the typical configuration of a spread spectrum communication system. This figure shows only baseband parts of the transmitting and receiving sides.
On the transmitting side, transmission data is information-modulated in a modulating unit 101. The modulation method to be used here is the PSK or the QAM, etc. A spreading unit 102 multiplies the information-modulated data with a PN (Pseudorandom Noise) code. Namely, the information-modulated data is further spread-modulated. The output of the spreading unit 102 is transmitted over a carrier wave.
On the receiving side, a despreading unit 111 multiplies the received signal whose frequency is converted into a baseband bandwidth with the same PN sequence as that used by the spreading unit 102. That is, the despreading unit 111 despreads (re-spreads) the received signal, and makes its state identical to that of the output signal of the modulating unit 101. The demodulating unit 112 demodulates the signal despread by the despreading unit 111, and regenerates the transmission data.
To improve the information transmission speed in the above described data transmission system, a multi-leveled number (the number of signal points) of information modulation is increased. If the number of signal points to be used for data transmission increases, the amount of data which can be transmitted in parallel increases. Therefore, the information transmission speed can be improved without changing the bandwidth of a transmission channel.
The method for improving the information transmission speed by increasing the multi-leveled number (the number of signal points) of the information modulation is applied not only to a system using a spread spectrum communication method, but also frequently applied to a normal data transmission.
If the multi-leveled number (the number of signal points) of the information modulation increases, the distance between each signal point becomes shorter as shown in FIGS. 2A through 2C. The distance between signal points plays a central role in determining the performance. As the distance becomes shorter, the error rate is reduced.
To prevent the error rate from being increased with an increase in the number of signal points, an S/N ratio must be improved. When a comparison between a 4PSK method and a 64QAM method is made, the S/N ratio must be improved by approximately 8 dB in order to attain the same error rate of 10.sup.-5 in a white Gaussian noise environment.
As the technique for improving the S/N ratio, the method using an error correcting code is known. The error correcting code is intended for adding redundancy to digital information in order to correct an error that has occurred in a communications channel when the digital information is transmitted. One example when an error correcting code circuit is simply added to the configuration shown in FIG. 1A is the configuration shown in FIG. 1B. In FIG. 1B, a convolutional coding unit 103 is arranged before the modulating unit 101 on the transmitting side, and a Viterbi decoding unit 113 is arranged after the demodulating unit 112 on the receiving side. With such a configuration, the error rate when data is transmitted can be improved, and the number of signal points can be increased to some extent, in comparison with the configuration shown in FIG. 1A. In this way, the information transmission speed can be improved.
However, if the error coding process is performed, the amount of data to be transmitted is normally increased by adding redundancy to sending data. Therefore, the data transmission rate must be raised in correspondence with that amount, so that the signal bandwidth becomes wider. Normally, the communications bandwidth of a radio communication is constrained. Using the error correcting code broadens the bandwidth, since the information amount to be transmitted is increased. However, a signal bandwidth is constrained. As a result, a spread gain (the ratio of a signal bandwidth before being spread to a communication bandwidth after being spread in the spread spectrum communication) may be constrained by broading of the signal bandwidth caused by the error correcting code.
FIG. 1C shows the configuration obtained by improving the configuration shown in FIG. 1B. In this configuration, a mapping unit 104 is arranged instead of the modulating unit 101, and a re-mapping unit 114 is arranged instead of the demodulating unit 112. The mapping unit 104 stores an input pattern and phase amplitude plane information used when a signal of that pattern is transmitted by determining a correspondence between them. It extracts the phase amplitude plane information by using the input signal (the output of the convolutional coding unit 103) as a key, and transmits the transmission data based on the phase amplitude plane information. The remapping unit 114 regenerates the transmission data by performing the process corresponding to that performed by the mapping unit 104, on the receiving side. This implementation may be sometimes called a coded modulation system.
With the coded modulation system shown in FIG. 1C, the error rate when data is transmitted can be improved without extending a signal bandwidth. However, considering a terrestrial channel environment in which multipath fading, etc. occurs, tolerance for the shortest distance between signal points cannot be said to be enough, and there is a limitation on improving the information transmission speed by increasing the number of signal points. Also, with the configuration shown in FIG. 1B, there is a limitation on improving the information transmission speed by increasing the number of signal points, considering a terrestrial channel environment in which multipath fading, etc. occurs.